It is a log of what was happening in the game when an error occured. The minidump is not the error itself as many people believe but rather an exception handling routine that fires when the error occurs to create the log file before exiting the game.
From it the devs are able to determine information about the state of the game when it had the error. When you send the file to them, the idea is they then use it to work out why your game had an error.
